As key components in digital computing and communications equipment, memory devices have been categorized as follows:
static random access memories (SRAM) are fastest (having cycle times in nanoseconds), at the expense of taking up more chip area and requiring more power than other types; PA1 dynamic random-access memories (DRAM) may be very dense (1 Gigabit chips are in development), they are fast (cycle times in tens of nanoseconds), but they are "volatile" and need to be periodically refreshed, as well as a permanent power supply to keep stored data; PA1 electrically erasable programmable read only memories (EEPROM) are "nonvolatile" in that they can store data without power supply for long periods of time such as a year or more, but writing data into such memories is slow (requiring microseconds).